<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
 
       input{
        color: #999;
       }

    </style>
</head>
<body>
    <input type="text" value="手机">

   <script>
        var text = document.querySelector('input');

        text.onfocus = function() {
            // console.log('得到了焦点');
            if(this.value == '手机'){
                this.value = '';
            }
        }

        text.onblur = function() {
            // console.log('失去了焦点');
            if(this.value === ''){
                this.value = '手机';
            }
        }


    </script> 

    
</body>
</html>